Brown trout Salmo trutta - Department of Agriculture, Forestry and ...

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Production (tonnes)Research into the impacts of habitat degradation and climate change on S. trutta survival arenecessary to determine future cumulative impacts. Also, the origins of trout populations i.e. whetherthey are naturally established or artificially restocked should be investigated to determine the trueextent of S. trutta invasion. Finally, further research would be useful regarding the ecological andsocio-economic impacts and viability of S. trutta land-based marine culture.9 Benefit assessmentThe Food and Agriculture Organisation of the United Nations (FAO) does not publish global annualstatistics on the international production and value of S. trutta (as exist for rainbow trout).In 2008, there were a total of 24 trout farms in South Africa (all registered as O. mykiss farms) (Britzet al. 2009). The total 2011 production of trout was 950 tonnes (DAFF 2012a), 95% of the troutproduction for aquaculture purposes in South Africa involves O. mykiss, with 5% production of S.trutta (K. Resoort, Malapong, pers. comm.). Based on this estimation, approximately 60 tonnes of S.trutta is produced annually. The growth of trout (all species) production in South Africa over the lastfive years has increased overall (with a marked decrease in 2007) (DAFF 2012a) (Figure 1). Troutfarming in South Africa was valued at ZAR27.9 million in 2008 (Britz et al. 2009).100090080070060050040030020010002006 2007 2008 2009 2010Figure 5.South African trout production (tonnes) 2006-2010 (Source: DAFF 2012a).South African trout farms in 2008 were employing 346 full time and 163 part time staff. Thesefigures are conservative as they include only those involved in primary production and not thosewho work in the secondary services (such as feed manufacturers or those employed in fishprocessing plants) (Britz et al. 2009).10 Risk Assessment10.1 Likelihood of this species becoming established in South AfricaBrown trout are reportedly already established in the upper reaches of many rivers and streams inSouth Africa (see Figure 3). Given the long history of this species in the country (present since 1890)Page | 8
and the considerable effort that has been invested in facilitating spread around the country, thereare probably few areas where self sustaining populations could become established where they donot already exist.Nonetheless, the invasive potential of S. trutta in the twenty ecoregions of the country (Figure 4,Table 1) that do not meet the known basic physiological tolerance criteria of the species wasassessed in accordance with the European Non-Native Species Risk Analysis Scheme (ENSARS)developed by CEFAS (UK Centre for Environment, Fisheries & Aquaculture Science) (Copp et al.2008). ENSARS provides a structured framework (Crown Copyright 2007-2008) for evaluating therisks of escape, introduction to and establishment in open waters, of any non-native aquaticorganism being used (or associated with those used) in aquaculture. For S. trutta, 49 questions wereanswered, providing a confidence level and justification (with source listed) for each answer. Theresults of the assessment on S. trutta can be found in Appendix 1.The outcome of the scoring was that S. trutta should be further evaluated before introduction in thetwenty remaining ecoregions. However, this was a conservative score for general S. trutta farming,and accordingly, we have adapted this result with each different culture technique in mind (section12).10.2 Potential ecological impactsEscapees from aquaculture facilities are inevitable and occur worldwide unless appropriatemitigatory methods are applied. A relative of the brown trout, the rainbow trout, have been foundto travel up to 360 km following escape from aquaculture facilities, and have a 50% chance ofsurvival, with some individuals surviving up to one year in the wild (Paterson 2010). Therefore, theimpacts of an introduction could potentially spread across an entire river catchment very rapidly.However, this study did not research the breeding potential of escaped fish so the impacts may onlybe transient.As there was no biological baseline survey done in South Africa in the 19 th century prior to the arrivalof S. trutta, it is very challenging (as with many species) to accurately predict the impact of browntrout introductions (Bartley & Casal 1998).Brown trout are currently though to have had an impact (either through predation or competition)on at least three of the 50 South African fish species which are listed as threatened in the Red DataBook (Skelton 1987, Cambray 2003b). The presence of S. trutta and Oncorhynchus mykiss have beenimplicated in the decline of amphibian species, Hadromophryne natalensis, in uKhahlambaDrakensberg Park World Heritage Site (Karssing 2010). However, it is unclear whether thesepopulations of trout are the result of farm escapees or the intentional release by anglers.The risk of S. trutta hybridising with another indigenous species in South Africa is considered to bevery low. There is low risk of the brown trout hybridizing with other congenerics as there are nonative Salmonidae in South Africa.High stocking densities commonly found in hatcheries can lead to outbreaks of parasites anddiseases, if the hatchery design and management is not well maintained. Some of the parasitesPage | 9
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